Epithelial Tissue: Structure, Function, and Classification

• Topic II covers ECM, Epithelium and Glands

Extracellular Matrix (ECM)

• Tissues and organs have cells and non-cellular components
• These organize into networks named the extracellular matrix (ECM).
• ECM comprises collagens, proteoglycans elastin, fibronectin, laminins, and glycoproteins.
• ECM components bind to each other and cell adhesion receptors
• This forms complex networks where cells reside in all tissues.
• Cellular functions like survival, growth, migration, and differentiation, are regulated by cell surface receptors transducing signals from ECM.

Composition and Assembly of ECM

• Involves epithelial cells, basement membrane, integrin binding, matricellular proteins, ECM fibre, cell and PG hydration.

Mechanism of Action of ECM

• It involves laminin fibers, collagen, fibronectin, growth factor and receptor, integrin, focal adhesion complexes, Actin cytoskeleton, nucleus, cytoplasmic signal transduction pathways, proliferation, differentiation, protein sysnthesis, attachment, migration and shape change

ECM Interaction via Integrins

• Both epithelial and mesenchymal cells: fibroblasts interact with ECM via intergrins
• Basement membrane features include Type IV collagen, Laminin and Proteoglycan
• Interstitial matrix features fibrillar collagens, elastin, proteoglycan and hyaluronan

Functions of ECM

• Anchorage
• Migration barrier and track
• Signal reservoir and presenter
• Changes in cell behaviour
• Low affinity corereceptor
• Biomechanical force
• Functional fragments

Histochemistry

• PAS reaction and hematoxylin are useful for ECM
• Carbohydrates like glycogen, proteoglycans, glycoproteins, and glycosaminoglycans are demonstrated
• Their presence is indicated by a red reaction product
• Such structures are termed PAS positive.

Classification of ECM

• ECMs vary in composition and structure
• Two types are interstitial and pericellular matrices.
• Interstitial matrices surround cells
• Pericellular matrices are in close contact with cells
• Laminins provide the substrate for binding of epithelial cells
• Epithelial cells are anchored to basement membrane via hemidesmosomes.
• Cells embedded into ECMs interact with this macromolecular network through their surface receptors such as integrins
• They integrate signals from ECMs affecting their functions
• All cell types synthesize and secrete matrix macromolecules under the control of multiple signals which participate in the formation of ECMs.

Epithelium

• Epithelium is comprised of a layer/sheet of cells covering a surface, lining a cavity/hollow organ, or constituting glands.
• Epithelia form membranes covering the body's surfaces, lining internal surfaces, or occurring as glands.
• Epithelia are separated from connective tissues by a basement membrane made of basal lamina and lamina reticularis.
• Functions include barrier, synthesis, secretion, absorption, and sensory reception.
• Can be freely accessible carcinogens

Classification of Epithelia

• Classification is based on the number of cell layers and cell shape.

Clinical Correlation

• Most cancerous tumors in adults originate from epithelial cells.
• These cells have constant cell renewal, a high mitotic index and exposure to mechanical stress/trauma.
• Due to close contact with the surface/lumen of the internal environment, pathogens and carcinogens have free access making carcinogens harmful.

Epithelial Membranes

• Epithelial membranes are avascular
• They can cover a surface or line a cavity/tube.
• Covered surfaces may be dry (outer body surface) or wet (ovary covering).
• Mesothelia, endothelia is based on anatomic location
• Classification is according to the shape of the superficial cell layer: squamous, cuboidal, or columnar

General Topography

• The number of cell layers Simple - one layer Stratified - more than one layer Variations in size and shape of cells Squamous (flat) Cuboidal Columnar
• The shape/size/position of the nucleus in relation to the cell shape.
• The staining reaction of the cytoplasm - presence or absence of granules
• Cell borders - whether distinguishable or not.
• Surface specializations - whether present/absent Microvilli (striated or brush border) Cilia Stereocilia.
• Keratinization - whether present/absent.

Modified Epitilials

• Microvilli
• Cilia

Columnar Cell with MIcrovilli

• Length varies between 0.5 - 1.0µm, diameter varies around 0.1 µm.
• Nonmotile
• Absorption
• Example is intestinal epithelium, proximal convoluted tubules of the kidney

Columnar Ciliated Cell

• Length varies between 5 - 10 µm, Diameter is approximately 0.2 µm.
• Motile
• 9 + 2 Pattern of microtubules
• Driving the entangled particles: transport in one direction
• Example is the respiratory tract, uterine tube, ependyma

Simple Epithelium

• Simple epithelia may be further classified according to the shape of the cells.
• Squamous epithelium
• Cuboidal epithelium
• Columnar epithelium
• An epithelium consists of one layer of cells

Multilayered Epithelia

• Also known as Multilayered Epithelia
• Includes Cuboidal, polygonal or rounded cells, stratified squamous and transitional epithelium
• Transitional epithelium is transitional: between unilayered epithelia and stratified squamous epithelium.

Simple Squamous Epithelium

• Simple squamous epithelium lines visceral cavities, lumina of vessels, and tubules
• It functions as an active barrier
• It also lines organs/mesenteries of thoracic and peritoneal cavities (mesothelium).
• In loop of Henle, air diffusion in lung, endothelium of blood, and water of lymphatic vessels.

Simple Columnar Epithelium

• Lines small/large intestines, stomach, major gland ducts, kidney convoluted tubules, gallbladder, small lung bronchi and male reproductive parts.
• They are found in protection of wet surfaces, absorption of nutrients, and secretion.
• Ovoid in shape, centrally/basally placed nucleus.
• Simple columnar has cells taller than they are wide.
• Microvilli increase surface area - thin, finger-like cellular projections.
• Brush borders consist purely of microvilli

Pseudostratified Epithelium

• Pseudostratified epithelium has more than one type of epithelial cell that vary in size, shape, and function.
• The basal layer is replacement (stem) cells with mitotic potential.
• The more apical nuclei layer are tall columnar cells which bear cilia.
• Mucous goblet cells are in this epithelium.
• Only some cells contact a basement membrane, only some reach the free surface.
• Metaplasia of respiratory epithelium to a stratified squamous

Stratified Squamous Keratinized Epithelium

• Acts as a protection against abrasion, dehydration and invasion of pathogens.
• The surface layer has dead cells devoid of nuclei and transformed cytoplasm into keratin plates
• It lines the tympanic membrane's outer surface, parts of the oral cavity, and mucocutaneous junctions.
• Basal and squamous cell carcinoma types are most common

Stratified Cuboidal and Stratified Columnar Epithelia

• Both contain 2+ cell layers, and are stratified
• They are better suited to withstand wear-and-tear
• Stratified cuboidal epithelium lines the ducts of sweat glands and other exocrine glands.
• Stratified columnar epithelium is in the pharynx/larynx, major ducts of exocrine glands, conjunctiva of the eyelids and parts of the male urethra.
• It also occurs at epithelial transition where it is interposed between 2 other types of epithelia.

Transitional Epithelium

• Multilayered epithelium in lower urinary tract.
• Adapts to change from a tall to a thinner epithelium with distention and contraction
• Umbrella cells are dynamic at the luminal surface.
• Transitional cell carcinoma (TCC) can occur, causing hematuria and pain from gradual obstruction.

Glands

• Most are formed by downgrowths of epithelial cells into surrounding tissues

• Exocrine glands: secrete products into ducts

• Endocrine glands: secrete hormines in blood

Classification of exocrine glands

• Classified by:
• Morphology of functional units
• Branching of Ducts
• Types of Secretory Products Manufactured
• Cell component method release of secretory products

Serous Cells

• Polarized secretory cells which produce watery proteinaceous secretion that contains enzyme
• The are found in secretory units of pure serous cells, exocrine pancreas and and lacrimal cells
• They contain Zymogen granules in the apical cytoplasm released via exocytosis

Mucous Cells

• Gives mucus protection and lubrication of surfaces
• Found either as Goblet cells, or in Groups organized as tubules or acini
• Mucus droplets are dissolved by out by histologic methods, the cytoplasm is typically pale staining Hematoxylin and eosin sections
• Major and Minor Salivary Glands that are either Pure Mucous or Seromucous

Multicellular Exocrine Gland

• Classified by shape and arrangment Tubular Acinar (alveolar) Tubuloacinar

• Type 1, Secretions may be simple, unbanched

• Type 2, Secretions may be compound

• Classified by mode of secretion

1. Merocrine : Use exocytosis
2. Apocrine : Found in mamary glands involves apical cytoplasm
3. Holorcrine Secreation

• Maligant tumore of Grandular Epitherlial is know as edenocarcinomas